Natural gas reservoirs may often contain high levels of acid gases, such as CO2. In these cases, a cryogenic process may provide an efficacious way to separate the acid gases from the methane. The cryogenic process could include a simple bulk fractionation, a Ryan-Holmes process, or a more complex cryogenic fractionation process. The cryogenic processes separate methane from CO2 by condensation and fractionation, and can produce the acid gas in a liquid phase for efficient disposal via pumping. However, in the cryogenic processes heavier hydrocarbons are separated with the CO2 in a single liquid stream. Often, the CO2 will be immediately reinjected for disposal, where the mixture will not cause any problems.
In some locations, a natural gas reservoir contains high levels of CO2. It is advantageous in these cases to use a cryogenic process to separate the CO2 from the methane. The cryogenic process could be simple bulk fractionation, a Ryan-Holmes, or a Controlled Freeze Zone (CFZ™) process. These processes separate methane from CO2 by condensation I fractionation, and can provide the CO2 as a liquid for efficient disposal. However, in these processes all hydrocarbons heavier than methane (C2+ or “ethane plus”) are also condensed and separated with the CO2. Normally, the CO2 will be reinjected for disposal, but the hydrocarbons are valuable and it is preferred that they be recovered for sale.
Separation of the heavier hydrocarbons can be performed by fractionation. However, ethane forms an azeotropic mixture with CO2, as discussed with respect to FIG. 1. The azeotropic prevents separation by normal techniques.
FIG. 1 is a temperature-composition phase plot 100 showing the equilibrium concentrations of CO2 in a mixture with ethane at 4,137 kilopascals (kPa, 600 psia). The x-axis 102 indicates the mole fraction of CO2, while the y-axis 104 represents the temperature in degrees Celsius (° C.). The concentration of the CO2 in the vapor phase 106 matches the concentration of the CO2 in the liquid phase 108 at about 70% CO2/30% ethane, as indicated by an arrow 110. This prevents separation-by-fractionation across the azeotrope (left to right, or right to left).
FIG. 2 is a temperature-composition phase plot 200 showing the equilibrium concentrations of CO2 in a mixture with ethane at 689.5 kPa (100 psia). Like numbered items are as described with respect to FIG. 1. As this plot 200 shows, concentration of the CO2 in the vapor phase 106 approaches the concentration of the CO2 in the liquid phase 108 at about 60% CO2/40% ethane, as indicated by an arrow 202. This prevents separation-by-fractionation across the azeotrope (left to right, or right to left). As these plots 100 and 200 indicate, complete separation by fractionation cannot be achieved without some additional separation processes.
Since the vapor and liquid compositions are equal at some point (70% CO2 at 4,137 kPa and 60% CO2 at 689.5 kPa), complete separation by fractionation cannot be achieved without some additional factor. Current practice for CO2 I ethane separation includes various methods. For example, a heavy component (lean oil) can be added, which preferentially absorbs the ethane. This is called “extractive distillation.” As another example, two-pressure fractionation can be used to exploit the small difference in the azeotropic composition between different pressures, for example, using two fractionators to fractionate at both 4,137 kPa and 689.5 kPa. This requires very large recycle stream, large fractionation systems, and is very energy intensive. Methods to exploit other physical and chemical properties (not dependent on vapor-liquid equilibria) can be used in conjunction with fractionation to achieve separation. These methods may include the use of amines in a chemical reaction with CO2, gas permeation membranes, or molecular sieves.
For example, U.S. Pat. No. 4,246,015, to Styring, discloses a method for separating carbon dioxide and ethane based on washing ethane from frozen carbon dioxide. The separation is accomplished by freezing the carbon dioxide in a carbon dioxide and ethane mixture and washing the ethane from the solid carbon dioxide with a liquid hydrocarbon having at least three carbon atoms. The freezing process may be preceded by distillation of a carbon dioxide-ethane mixture to form an azeotropic mixture. A subsequent distillation may be used to separate the wash hydrocarbon from the carbon dioxide. In addition, if desired, the ethane-wash hydrocarbon mixture may be similarly separated in a subsequent distillation stage.
U.S. Patent Application Publication No. 2002/0189443, by McGuire, discloses a method of removing carbon dioxide or hydrogen sulfide from a high pressure mixture with methane. The high pressure gas is expanded through a flow channel having a convergent section followed by a divergent section with an intervening throat which functions as an aerodynamic expander. The flow channel is operated at temperatures low enough to result in the formation of solid carbon dioxide and solid hydrogen sulfide particles, which increases the efficiency of carbon dioxide and hydrogen sulfide removal.
International Patent Publication No. WO/2008/095258, by Hart, discloses a method for decreasing the concentration on carbon dioxide in a natural gas feed stream containing ethane and C3+ hydrocarbons. The process involves cooling the natural gas feed stream under a first set of conditions to produce a liquid stream of carbon dioxide, ethane and C3+ hydrocarbons and a gas stream having a reduced carbon dioxide concentration. The liquid stream is separated from the gas stream, and C3+ hydrocarbons may be separated from the liquid stream. The gas stream is then cooled under a second set of conditions to produce a sweetened natural gas stream and a second liquid containing liquid carbon dioxide and/or carbon dioxide solids. The sweetened natural gas stream is separated from the second liquid.
International Patent Publication No. WO/2008/084945, by Prast, discloses a method and assembly for removing and solidifying carbon dioxide from a fluid stream. The assembly has a cyclonic fluid separator with a tubular throat portion arranged between a converging fluid inlet section and a diverging fluid outlet section and a swirl creating device. The separation vessel has a tubular section positioned on and in connection with a collecting tank. A fluid stream with carbon dioxide is injected into the separation assembly. A swirling motion is imparted to the fluid stream so as to induce outward movement. The swirling fluid stream is then expanded such that components of carbon dioxide in a meta-stable state within the fluid stream are formed. Subsequently, the outward fluid stream with the components of carbon dioxide is extracted from the cyclonic fluid separator and provided as a mixture to the separation vessel. The mixture is then guided through the tubular section towards the collecting tank, while providing processing conditions such that solid carbon dioxide is formed. Finally, solidified carbon dioxide is extracted.
Each of these methods presents a drawback. For example, using a lean oil contaminates the ethane, and requires large amounts of heat, for regenerating the lean oil. Further, large lean oil circulation rates are needed and the technique does not allow complete ethane recovery. Two-pressure fractionation systems require very large recycle streams and equipment sizes, increasing costs. Techniques that use amines, membranes, and mole sieves all release the CO2 as a vapor at low pressure, increasing the cost of disposal. Finally, the expander separation devices generate the CO2 as a solid. Thus, there is a need for a better method of separating CO2 and ethane.